Studies using magnetic resonance imaging (MRI) as a non-invasive alternative to cardiac catheterization for the imaging of coronary arteries have been presented by several research groups. Despite the preliminary demonstration of feasibility of magnetic resonance coronary angiography (MRCA), it is not used clinically for routine evaluation of patients with coronary artery disease. We have recently shown that using a navigator echo image as a real-time respiratory gating device can produce images of coronary arteries which may allow there use in the clinical arena. However, there are also concerns and questions about the impact of navigator echo gating on the accuracy of the images obtained. Specifically, the appearance of stenoses on navigator echo gating images remains uncertain. One area where MRCA could have an immediate impact in the clinical arena is in the evaluation and management of restenosis after percutaneous transluminal coronary angioplasty (PTCA). The overall goal of this research is to develop navigator echo driven magnetic resonance coronary angiography and coronary low measurement into a useful tool for detecting restenosis after PCTA. The specific aims of the study are: 1) Conduct model studies on realistic stenotic coronary flow phantoms with respiratory motion and pulsatile flow to: a) compare 2D and 3D acquisition methods and optimize the most promising approach, b) determine the impact of multiple signal average, high- resolution, navigator echo-driven imaging on the detection of stenose, c) investigate coronary hemodymanics and assess whether stenosis severity can be estimated from post-stenotic signal loss plus flow velocity measurement, d) evaluate the impact of intravascular contrast agents on determining stenosis severity, e) determine the precision and accuracy of an optimized navigator echo-driven MR flow measurement. 2) Assess the accuracy and clinical utility of the optimized MRCA technique for detection of restenosis in post-PTCA patients. Examine 50 patients pre- and post-PTCA with: a) the optimized, multiple signal average, high-resolution, navigator echo-driven MR coronary angiography sequence and compare the images to x-ray angiography, and b) the navigator echo driven MR phase velocity sequence and compare the results to measurements made with intravascular ultrasound.